2:12 AM, Sunday March 28th 2021
Personal view is that would be a waste of a good sketchbook. However there are practical reasons for using printer paper too. The finish of printer paper works well with the fineliners. You get crisper lines than perhaps you would with sketchbook paper designed for softer mediums like graphite or pastel. Using cheap paper also means you worry less about wasting paper which can inhibit your confidence when learning. This could be a real detriment to your progress.
I went through a lot of paper so far. I have passed lesson 4 and have used hundreds of sheets for practice. I enjoy the process of doing the practice and putting the practice pages into recycling. It felt I was absorbing the practice and moving on.
That's my experience. Maybe Uncomfortable has a different view.